Update NavigationObstacle API doc

[smix8]

Updates NavigationObstacle API doc.

Godot 4.1 only.

Update for when PR godotengine/godot#78081 gets merged.

[mhilbrunner]

One consideration is that everything in code blocks is not translated for docs translations.

It may make sense to have "For 2D"/"For 3D" formatted above the code blocks as small headers or even just bolded text, what do you think?

Likewise, prefer keeping descriptions of what the code is doing next to the code instead of as comments in the code, although thats a case-by-case (or comment-by-comment) decision, short comments are generally fine :)

[smix8]

I would preferred "extends Node2D/3D" in the first line to signal that the code snippet is for 2D or 3D because it creats a highlight color in the code instead of they greyed comment color. There are users regularly that do not understand this. They think the code snippet is broken because they try to do a direct 1:1 copy of the code into their projects.

Not sure, most comments, when placed next to the code, become hard to read due to line length/wrap in my browser.

Maybe there should be a special syntax for code-snippets lines that adds them to the translation strings.

[skyace65]

Thanks!